Influence of the applied field frequency on the characteristics of Ar and SF6 diffusion plasmas sustained at electron cyclotron resonance above multipolar magnetic field structures

In argon plasmas excited at electron cyclotron resonance above multipolar magnetic field structures, ion density increases linearly with microwave input power but saturates as it gets near the critical density, This behaviour is observed at the three microwave frequencies investigated, namely 960 MHz, 2.45 GHz, and 5.85 GHz, as well as for different magnetic field configurations. The saturation density is independent of the atomic or molecular nature of the gas, as shown with Ar and SF6. Expectedly, the ion density saturation value varies as the square of the excitation frequency, while the microwave input power required to reach saturation is proportional to the critical density, For a given multipolar magnetic field confinement, the electron temperature is shown to decrease with increasing excitation frequency, This result stems from the confinement of the fast electrons, which generate the plasma, The evolution of the F-atom concentration in SF6 discharges, as measured by actinometry, is observed to saturate with microwave input power at values depending on gas pressures at both the 2.45 GHz and 5.85 GHz excitation frequencies. Ion density and F-atom concentration saturate at distinct microwave input powers.